THE RADAR RECEIVER lU 



interference by the neck of the tube with the deflected beam such off-center 

 display deflecting structures are mounted close to the junction of the neck 

 and bulb of the cathode-ray tube. 



As indicated previously the blue-flash characteristic of the excitation of 

 the first layer of the cascade-type screen is usually reduced in intensity 

 through the use of an optical filter placed between the screen of the cathode- 

 ray tube and the observer. These optical screens are usually constructed of 

 an amber-colored transparent plastic whose particular optical transmission 

 characteristics are chosen in accordance with the particular phosphor and 

 speed of scanning employed. It is common practice to engrave general 

 range and direction reference lines on such screens, and in many cases 

 variable edge lighting of these engraved screens is employed to enhance the 

 display. 



In certain applications of radar, namely, airborne reconnaissance and 

 bombing operations, it is desirable to obtain a photographic record of the 

 display to be used for training, briefing, or damage assessment purposes. It 

 is customary in these cases to employ a stop-frame moving picture camera 

 attached to the indicator and exposed periodically as desired. Figure 43 

 indicates the design of a radar indicator viewing attachment which was 

 employed in connection with an airborne bombing radar equipment toward 

 the end of the past war. In this design a partially silvered mirror is located 

 at a 45° angle with respect to the axis of the cathode-ray tube. An illumi- 

 nated image of an adjustable course marker located below the mirror is 

 observed superimposed upon the radar presentation with negligible parallax 

 distortion. A portion of the light of the radar image is also reflected from 

 the surface of the partially silvered mirror, and together with the direct 

 image of the course marker is available for photographic recording by the 

 camera mounted as shown. Automatic exposure at any preselected time 

 interval is provided, the exact exposure time being controlled by impulses 

 derived from the azimuthal scanning mechanism of the radar antenna. The 

 photographic recording of radar displays has become a matter of prime im- 

 portance in the modern military operations and has added another consid- 

 eration for the military radar indicator designer. 



Figure 44 indicates an equipment arrangement of a PPI indicator as 

 employed in the AN/APQ-13 radar bombing system. In this system the 

 indicator is designed for convenient overhead mounting in the restricted 

 radar operating space available in modern bombing aircraft. The deflection 

 coil in this equipment is rotated about the neck of the 5" diameter long- 

 persistence cathode-ray tube by a geared selsyn motor energized from a 

 similar selsyn unit mechanically linked to the rotating radar antenna. Per- 

 manent magnet focussing and centering is employed in this particular indi- 

 cator. Figure 45 illustrates a somewhat similar packaging arrangement for 



